MICROSCALE TORTUOSITY AND ITS VARIATION AS OBSERVED IN TRIGGERED LIGHTNING CHANNELS

Slow-streak photographic recordings of the lowest approximately 4-5 m of lightning channels in four triggered flashes are presented to docum ent examples of microscale tortuosity variations (similar to 10 cm or smaller geometric features). These recordings have both high spatial r esolution (centimeter scale) and modest temporal resolution (similar t o 1.5 ms) such that the channel geometry is accurately rendered formos t return strokes and their ensuing M-component events. The M-component events, verified as such from ancillary fast-streak data records, oft en show a significant enhancement of the microscale tortuosity relativ e to the previous return stroke channel. Channel length increases (two -dimensional) corresponding to the tortuosity enhancements are found t o be as large as 55%, though the mean for 10 evaluations is 23%. Zn co ntrast, two M-component events show a marked overall reduction of the microscale tortuosity relative to an immediately preceding sequence of several discharge pulses along the same meter scale channel. Ensuing return strokes occurring along common but previously distorted channel s also show a tendency for overall tortuosity reduction or ''straighte ning.'' These tortuosity variations appear to be unrelated to the pres ence of the vaporized trigger wire residue. Three primary mechanisms o f tortuosity enhancement are examined: Z pinch magnetohydrodynamic ins tability (MHD), discharge generated turbulence, and ambient planetary boundary layer turbulence. The Z pinch instability is assessed quantit atively and may be a contributor through either the high current phase or integrated over a continuing current period. Three variants of dis charge generated turbulence also could be contributors, but a critical test of these mechanisms cannot be made with the present data set. Am bient planetary boundary layer turbulence appears unable to account fo r the observed tortuosity enhancements. An inductive mechanism of tort uosity reduction is also examined as a possible explanation for the di sappearance of individual acute kinks or loops as well as overall chan nel tortuosity reductions. This mechanism cannot explain M-component r elated tortuosity reductions, but it might be a viable explanation for return stroke straightening given the much larger current derivatives involved. A full accounting of microscale tortuosity variations in li ghtning discharge channels remains to be achieved.